1. Technical Field of the Invention
The present invention relates to a projector for projecting light supplied with information such as an image or the like onto a screen to display the image on the screen.
2. Description of the Related Art
A projector using a liquid crystal panel or DMD (digital micro-mirror device) has been known as a projector for projecting light supplied with information such as an image or the like to a screen to display the image on the screen. According to a liquid crystal projector using a liquid crystal panel, light irradiated to the liquid crystal panel is transmitted through the liquid crystal panel or light irradiated to the liquid crystal panel is reflected by the liquid crystal panel, whereby image information displayed on the liquid crystal panel is projected to the screen. The image information displayed on the liquid crystal panel is displayed on the screen while being enlarged.
The liquid crystal projector is equipped with an illumination device for illuminating light to the liquid crystal panel, and an illumination optical system for uniformly irradiating light from the illumination device onto the liquid crystal panel is provided in front of the illumination device. The illumination optical system contains a lens, a polarization converting element, etc., and the light irradiated from the illumination device is irradiated through the illumination optical system to the liquid crystal panel. It is preferable that the liquid crystal panel is illuminated with bright and uniform light, and thus it is desired that the light irradiated from the illumination device has high brightness and the light flux thereof is uniform. Accordingly, it has been general to use as the illumination device a high-luminance discharge lamp which can light having brightness (high luminance) needed to project an image on the liquid crystal panel such as an ultra-high pressure mercury lamp, a metal halide lamp, a xenon lamp or the like.
The high-luminance discharge lamp is heated, and thus it is necessary to provided a large-scale cooling device for cooling the lamp. However, when the cooling device is provided, there is a problem that the illumination device must be designed in a large size and further the manufacturing cost of the projector is increased. Furthermore, it has been required to reduce the cost when the projector is used, for example, to reduce the power to be consumed to irradiate high-brightness light and further to lengthen the period for which the high-luminance discharge lamp can be used.
Therefore, it has been recently considered that a light emitting diode (hereinafter referred to as “LED”) is used as an illumination device in place of the ultra-high discharge lamp. LED has advantages that it is more compact in size, lighter in weight, smaller in power consumption and longer in lifetime, it can be driven with a low voltage and it has a high response speed when it is subjected to turn-on control as compared with the ultra-high discharge lamp described above. However, light emitted from LED is diffused over a broad range, and thus JP-A-2003-186110 has proposed a method of focusing light emitted from LED by a lens. Furthermore, a while-color LED for emitting white light is used as LED used for the illumination device.
There are some types of LEDs for emitting white light. One of these types of LEDs comprises a combination of an element for generating ultraviolet light and a white fluorescent material for emitting light upon reception of ultraviolet light (a mixture of plural kinds of primary color fluorescent materials). With this type, the whole ultraviolet light thus generated is not necessarily converted to white light, and a part of the light may leak to the outside. Even in other types of LEDs, leakage of ultraviolet light is unavoidable in some constructions. Furthermore, orientation film of the liquid crystal panel is weak to ultraviolet light, and the lifetime of the present liquid crystal panel is greatly affected by a total irradiation amount of ultraviolet light. Accordingly, when a liquid crystal projector is constructed by using LED as an illumination device, it is important to treat ultraviolet light thus leaking, and the JP-A-2003-186110 and other techniques have hardly paid attention to the above matter.